The invention relates to a method for growing monocrystals from shards of already-grown crystals, to crystals obtained in this way, and to their use as optical elements, especially in photolithography.
The production of crystals, especially for photolithography, is widely known and is described for instance in European Patent Disclosure EP-A 1 259 663. In it, large-volume monocrystals with a uniform orientation are grown from a melt of crystalline raw material, by slowly cooling the temperature of the melt at the bottom of a melting and growing vessel to the temperature of crystallization. Between the bottom of the melting vessel and the opening, a vertically extending axial temperature gradient is applied, and an addition or outflow of heat passing through the side walls is avoided. The crystal orientation is predetermined by means of a germ crystal, which is located in particular in a cooled germ pocket. In the growth and further processing of such crystals into optical elements, the crystals are comminuted, which produces a large quantity of material in the form of waste, breakage, or even defective growths, hereinafter called shards. Until now, these shards have been disposed of, even though they are not toxic, and this increases the production costs.
The attempt has already been made to re-use shards from growing calcium fluoride monocrystals, by evaporating them at a temperature of at least 1100° C. in a vacuum of at maximum 5×10−4 mbar and condensing them again at a temperature of between 500° C. and 1280° C. Such a method is described in German Patent Disclosure DE 10 2004 008 749 A1.
From glass production, it is known to include shards and breakage in the melt and deliver them to a new use. This not only serves to economize on raw materials but above all also serves to save energy that is otherwise needed for purification and for developing the chemical reactions that then lead to the finished glass melt. In the production of crystals, however, such energy savings are not attained, so that the energy expended for melting the high-purity raw crystal powder used until now and for melting shards is the same. The use of shards to economize on raw materials, however, involves the risk of contamination, so that in complicated, expensive, lengthy growing processes, until now it was preferable to use expensive, high-purity starting materials with a purity of >99.99%, rather than to reprocess such shards.